project presentation

Bolivar Roads Surge Barrier: How to estimate the optimal barrier height after a Risk Assessment 1 Katerina Rippi

Transcript of project presentation


Bolivar Roads Surge Barrier:How to estimate the optimal barrier height after a Risk Assessment

1Katerina Rippi

Conception of Ike Project

Existing Levee system

Proposed floodgate in Bolivar Roads passIke Dike2

Galveston Bay regionWhy protecting Galveston bay area is urgent?

6 million population and growingNationally-important population and economic centerSupports the largest petrochemical complex in the US (second largest in the world)Port of Houston alone generates over $178 billion in economic activity annuallyGalveston Bay provides valuable ecosystem services



Galveston Bay regionYearHurricane CategoryDamage [$]1900Galveston Hurricane420 million1957Audrey4147 million1961Carla5325 million1971Edith525 million1983Alicia32.6 billion2005Rita512 billion2008Ike430 billion



AliciaSome of the most catastrophic hurricanes in Galveston Bay region4

Galveston Bay region

(source: Keim & Muller, 2007)

One of the most frequently exposed regions to hurricanes5

Design considerationsLocation of the barrierType of barrier (sector, barge gate, caissons etc) Afloat or not?Open or (partially) closed structure?6

The storm surge barrier will consist of two sections; a navigational section and a section for refreshing the Galveston Bay in normal conditions.

Firstly the shortest path from the Galveston Island to the Bolivar Peninsula can be an appropriate location for the storm surge barrier. But considering the subsoil and depth along the alignment it may be better to position the barrier deeper inside the bay. A drawback of locating the barrier more inside the bay is that an modifications to the entrance channels to Galveston could be required. Other factors play a role as well, such as the impact on environment and shipping but also the amount of exposure of waves from the Gulf of Mexico. Deal with negative hydraulic heads (barge gate is preferable)3) Afloat: cope with high stream velocities, less vulnerable for negative hydraulic heads (the gate simply swings open again), if gate is set down additional measures are required such as (larger) valves, skirts and fenders, this discharge decreases the positive effect of the storm surge barrier on the water levels behind the barrier. But: the dynamic forces in the gate are higher and the wear and tear on the bearings are higher if the gate stays afloat.4) If a partially closed environmental section of the barrier provides sufficient flood protection the navigational section could remain open (without barrier). This section will have to deal with large flows during hurricane conditions and will therefore require a large bottom protection. It is noted that keeping a part of the barrier completely open can harm the image of the storm surge barrier in the perception of the general public. 6

Bolivar Roads Surge BarrierWhy a storm surge barrier?


Bolivar Roads Surge Barrier Objective of this study: What is the optimal height of the storm surge barrier in terms of cost efficiency and damage prevention inside the basin?

Sub-questions:What is the influence of local wind setup and inflow on increasing the water level inside the basin under hurricane conditions for different barrier heights?What are the water levels caused by different barrier heights inside the basin?What is the extend of the damage under different inundation levels?What is the risk reduction of a certain water level for different barrier heights?How can the damages and the initial investments for the construction be quantified? 8


General Methodology9


FundamentalsHurricane PrinciplesA hurricane is a type of tropical cyclone, which is a generic term for a low pressure system that generally forms in the tropics. The cyclone is accompanied by thunderstorms and, in the Northern Hemisphere, a counterclockwise circulation of winds near the earth's surfaceMaximum sustained winds of 74 mph (33 m/s) or higherForerunner component


FundamentalsLandfall location11

The hurricane's right side (relative to the direction it is travelling) is the most dangerous part of the storm because of the additive effect of the hurricane wind speed and speed of the larger atmospheric flow

Therefore, the hurricane track that is going to be considered in this study in order to estimate, the water level inside the basin, will be west of the basin. Besides, the regions of the Galveston bay that have the highest potential of damage and are the most social and physical vulnerable 11

FundamentalsReturn period of the storm surge in the Gulf side (Stoeten, 2013)



Report Galveston Bay (TU Delft, Iv-infra, Royal Haskoning): Design, Flood riskDeVries, 2014: Design of the environmental sectionStoeten, 2013: Risk Reduction for Galveston BayKarimi, 2013: Design of the navigational section

Master Project, 2014: Ike Dike in Bolivar Peninsula

13Preliminary research

Report Galveston Bay: proposed the location of the barrier, the division into 2 sections (navigational+environmental), characteristics of the barrier (dimensions, open or closed navigational section etc)De Vries: an equal height for both navigational and environmental part is preferable (more cost efficient)Stoeten: return period of storm surges, constructing a coastal spine is more risk averse that upgrading Texas levees or ship channels leveesKarimi: design navigational part, costs, proposed a research for a concrete navigational section rather than steel 13

FundamentalsArea Characteristics Bathymetry


FundamentalsMeteorological data

Dominant wind directions: south, southeast

Average rainfall over the Galveston Bay Area over 48 hours during Ike amounts about 200 mm, resulting in just a slight increase in water level in the Galveston Bay that does not coincide with the peak storm intensity. 15

Rainfall can be neglected. 200 mm in comparison with the wind setup and inflow effect which can cause much more amplification of the water level inside the basin in just an hour or so.15

FundamentalsPhysical Risk and Social Vulnerability16

FundamentalsExisting Protection Galveston island seawall Tide barrier in city of Kemah (100-yr event) Diversion channel and dam in the City of La Marque (500-yr event) Levee system in Texas City (500-yr event)


Existing Levee system

HazardSurge development inside Galveston BayApproximations Inlet: narrow channel with a zero storage capacity Basin: semi-enclosed, quasi steady response, constant depthMain components of the basin water elevation during hurricane eventOverflow only over the storm surge barrier. Overflow over barrier islands neglected due to Ike Dike full retaining height Local wave setup neglected : fetch and depth limitedRiver inflow: river discharging negligible, tide dominance (Ruijs, 2011)


HazardSimulation model Variables definition19

Discharge Q was divided into discharge over the barrier Qw (equation from weirs) and discharge through an entire open navigational section, Qc (Chezy equation)19

HazardAssumptionsPrevailing wind direction south/southeast, constantLandfall location at the west side of Galveston bayThe mean bay level increases solely due to inflow from the storm surge barrierWestern shore-normal winds effect on the bay Linear behavior of wind velocity and storm surge level in the gulf side


2) West sided landfall location as it is the worst case in terms of the induced damage3) No inflow from the barrier islands-Ike Dike is considered to fully block the surge 4) According to Sebastian et al. (2014) conclusion that during Ike hurricane the rising water levels in the west part of the bay were driven by the combined effect of counterclockwise, western shore-normal winds in the bay and shore-parallel winds along the Louisiana-Texas (LATEX) shelf. An increase of 30% in water levels has been implemented. 5) The reason the maximum wind speed does not coincide with the peak surge is because according to Harris (n.d.) and Pore (1964), peak surge was observed to coincide with alongshore wind components rather than onshore components. 20

AssumptionsCumulative distribution function of the storm surge (Generalized Pareto distribution )Relationship between maximum storm surge and maximum wind velocity (NOAA, Introduction to Storm Surge)


HazardMonte Carlo A broad class of computational algorithms that rely on repeated random sampling to obtain numerical results of a variable. Consequently, a probability distribution of a stochastic variable can be estimated from the simulated values. 22

Probability distribution of the water level inside the basin for each barrier height

In each Monte Carlo iteration, a random value was given in the extreme storm surge elevation (hocmax) according to their cumulative probability distributions for a certain probability of occurrence while the maximum wind speed (U10) was computed by a linear relationship between wind speed and storm surge level. Then, for different barriers heights from 0 to 6 meters, the water level in the west part of the basin is derived after an iteration loop of time steps equal to the time of closure of the barrier.22

Halfway conclusions 23Water elevation seems to converge after barrier height of 3.5 m which suggests that after this height the influence of the barrier inside the basin tends to be almost the same (see right figure). Almost linear relationship between water level inside the basin and the maximum occurred storm surge

Left figure: Water elevation inside the basin as a function of storm surge for 3 different barrier heights (1 m, 3 m and 6 m) for 4000 realizations. Right figure: Water elevation as it has been generated from 4000 realizations for barrier heights between 0.5 m and 6.5 m (on each box, the central mark is the median, the edges of the box are the 25th and 75th percentiles and the whiskers extend to the extreme data points). Both graphs were made for the 32th hour of closure where the peak surge meant to be appeared

Certainly, the fact that a linear increase and decrease of storm surge were assumed over time may affect their relationship. However, it is expected that as long as the maximum storm surge becomes bigger so does the water level inside the basin23

Halfway conclusions 24Local wind setup dominates inflow Inflow tends to decrease almost exponentially from lower to higher barrier heightsWind setup increases from lower to higher heights

Inflow tends to decrease almost exponentially from lower to higher barrier heights while the wind setup increases from lower to higher heights. This can be explained from the fact that a larger barrier height prevents a certain amount of inflow which incurs lower mean water levels than a lower barrier height. On the other hand, wind setup is inversely proportional to water depth and subsequently lower water levels lead to higher wind setup. This lead for barrier heights between 3 and 6 m to generate almost the same water level inside the basin.

The effect of the western shore normal winds was taken into account. An allowable inflow could increase this effect as it increases the water depth. However, this effect and its magnitude can only be determined after a detailed hurricane simulation with several landfall locations and intense. Probably for another landfall location other than west sided, this effect might be negligible. 24

Halfway conclusions Higher barrier leads to a lower water level inside the basinCDF tends to be the same for barrier heights above 3 m

Probability of exceedance (or hazard) is almost the same for barrier heights above 3 mFor events with a large return period, reduction of the probability of exceedance is larger for higher barriers but with a slight difference in comparison with lower barriers


CDF = Generalized Pareto, the probability of exceedance for heights between 4 and 6 m tends to be almost the same. This leads also to similar risk reduction as we will see later on.Hazard reduction = reduction in the probability of exceedance of a specific event9) This may be attributed to the number of iterations that have been carried out (4000 iterations). Probably, with more realizations the difference in the hazard reduction of different barrier heights would be obvious also for extreme events with return periods up to 1/10,000 yr-1 25

Halfway conclusions Probability of exceedance reduction 26

Halfway recommendations3D simulation model

FEMA cumulative probability function for the storm surge in the open coast

Flow under the barrier

Sediment allocation inside the basin (basin environment)27

Account for the more complex form of a hurricane (circular motion, change in wind speed over space and time) and basins bathymetry and coastal attributes (for example bathymetry)What is more, the cumulative probability function of the storm surge in the gulf side that was used as an input to the calculations, was this one estimated by Stoeten (2013) synthetic model simulation. However, FEMA has proposed different return periods that should certainly be investigated and included in the any similar research.


ConsequencesInundation map (MEOW)28

MEOW= Max Envelope of Water (Stoeten, 2013)28

ConsequencesExisting protection level29LocationInundation when: (estimated)Estimated Safety LevelGalveston (bay-side)WL > 1 m+MSL25-50 yr-1Galveston (ocean-side)WL > 4.5 m+MSL100 yr-1Texas CityWL > 5 m+MSL100 yr-1Houston Ship ChannelWL > 4 m+MSL100-500 yr-1Kemah (west side)WL > 2 m+MSL25-50 yr-1

From Jonkman et al. (2013). The estimated safety level was estimated from Kasper Stoeten (2014)29

ConsequencesExposure & VulnerabilityResidential (Single Family Residential 2-story structures)30

Data on land use was obtained through TNRIS (2013) while the US Census (2010) provides estimates on median home value within Census block groups.30

ConsequencesExposure & VulnerabilityResidential


Estimated damage for a 1/100 yr-1 event on the Galveston West End and Bolivar Peninsula are assumed equal to zero because of existing FEMA BFE requirements. The estimated direct tangible flood damage for a 1/100 yr-1 event is lower than observed after Hurricane Ike because it is assumed that lost or severely damaged property was rebuild to contemporary building standards.


ConsequencesExposure & VulnerabilityIndustrial


The Port of Houston is the second largest port in the United States with an estimated direct economic impact of 178.5 billion dollars a year (Martin Associates, 2011). Its largest asset is the chemical and petrochemical industry with a total crude oil processing capacity of 1,120,000 bpd, almost 10% of thenationwide capacity. The Port of Houston suffered minor damage during Hurricane Ike as surge levels remained approximately 0.6 meter below the docks (Bedient, 2012).Texas City is located about 40 kilometers South of Houston and home to three oil refineries with a combined processing capacity of 707,000 bpd (EIA, 2013). Texas City did not suffer flood damage during hurricane Ike, however the Texas City Dike was overtopped.32

ConsequencesExposure & VulnerabilityIndirect damage in million $US33Return period [1/yr]1/101/1001/1,0001/10,000IndustrialPort of Houston 026,00060,000120,000Texas City04,50016,00020,000PublicCritical infrastructure1,9002,8503,6103,800Transportation & Evacuation5378051,0191,073Navigation & waterways3,2004,8006,0806,400Agriculture and fisheries6009001,1401,200Total6,23739,85587,84915,2473

(Perry, et al., 2008) (Stoeten, 2013) (Master Project, 2014) The U.S. Coast Guard estimates that a one month closure of the Port of Houston will cost the national economy $60 billion dollar (USCG, 2013). Studies indicate that a five week disruption at a large oil refinery adds about $US 5,000/bpd to nationwide refined product expenses (CPRA, 2007). Additional losses relating to loss of sales and earnings amount to $US 12,000/bpd per five weeks ofdowntime (CPRA, 2007).Rebuilding oil refineries, docks or other industrial areas may take months. It is assumed that a 1/10,000 yr-1 event results in a 2-month disruption attributable to flooding. Short and thereby relatively inexpensive when compared to rebuilding efforts after Hurricane Katrina, which lasted up to one year (Admiraal, 2011). Indirect tangible damage figures of the 1/1,000 yr-1 and 1/5,000 yr-1 relate to the 1/10,000 yr-1 event by expressing them as a percentage of the direct damage.

Public damage related to the damage after hurricane Ike33

ConsequencesTotal estimated damage34

Flood Risk Assessment35





Probability of exceedance


Risk Assessment xPrinciple of Risk Assessment

!!!!!!!!!!!Hazard is different for each barrier height35

Flood Risk Assessment36To assess the performance of the risk reduction of different barrier heights, a Cost-Benefit Analysis (CBA) methodology is carried out. A cost-benefit analysis estimates the present day monetary value of cost and benefits of an intervention and provides a measure of how well a project performs over its life time. CBA

r discount rate and g growth rate 36

Flood Risk Assessment37

PiSCost Benefit Time Line

We consider that total investment costs occur directly after the construction of the structure. Benefits are defined as the damage avoided in comparison with the do nothing alternative.ssb// maintenance costs of the storm surge barrierMid// maintenance costs of the Ike Dike37

Flood Risk AssessmentDiscount and Growth rateInterest rate 7%Growth rate 5%


Per Capita Personal Income in Harris County, TX (source: Economic Research, 2014)

19962006Land use change Light green areas in the right picture indicate the development gain until 2006

Growth rate for Galveston bay is about 4.6% as well (3 counties are present in Galveston bay: Harris, Galveston and Chambers. The last one is excluded as its contribution to the growth is negligible and it is located in the east part of the bay while our consideration is limited to the west part)38

Flood Risk AssessmentConstruction and Maintenance costsStrom Surge Barrier in Bolivar Roads39

Maintenance 1% of the initial investments per year (maintenance every 10 yr)

C// cost of the barrierB//width of the barrierH//retaining height of the barrierh//maximum head across the barrier (for simplicity, it was considered to be equal to the water level outside the barrier. More precisely, the head is the difference between the water level outside and inside. It determines more or less the costs for the width and the foundation of the barrier. However, the water level inside the basin changes every hour under hurricane conditions and under away that needs investigation in order to come up with a reasonable result).This may underestimate costs. A negative head is also possible! Cunit// unit cost [$/m3], Karimi: 18,900 $/m3 for navigational, De Vries: 15,000 $/m3 for the environmental part, Van de Toorn: 30,000-40,000$/m3. The unit cost that was taken in this research was a combination of those estimations. More specifically 30,000 $/m3 for the navigational and 20,000 $/m3 for the environmental. This unit value takes also into account maintenance costs discounted (as I saw from the references that I took them from). They are thought to be considered reasonable values after an overview of similar projects and after a discuss with Iv-infra structural department. However, this indeed depend on the maintenance costs and their more precise elaboration. A well constructed barrier needs maintenance every 20 years while for the dike a reassessment of the safety level takes place every 6 years. Of course, there are also unpredictable maintenance costs such as after a severe hurricane strike which may have a dramatic impact on the structure.From a brief overview of maintenance costs, they may rise the barrier costs up to approximately 1 billion $ for 1/10,000 yr-1 event and 0.5 billion for a 1/100 yr-1 event. Which again shows that a safety level of 1/10,000 yr-1 maybe relatively beneficial. But, according to the aforementioned method, if we want to add the maintenance costs we should decrease the unit cost respectively.


Flood Risk AssessmentConstruction and Maintenance costsIke Dike (Master project, 2014)40Return period [1/yr]1/101/1001/1,0001/10,000Bolivar Peninsula [million $]311418582696Galveston island [million $]217292406486Total [million $]5287109881182

Maintenance costs 1.5% of the initial costs every 5 years

Maintenance is considered to take place every 5-6 years 40

Flood Risk AssessmentObtaining a sense of maintenance costs For a 100 yr lifetime and a maintenance procedure to take place every 10 years for both the storm surge and the land barrier 41Discounted maintenance costs [million $]/ Safety level [yr-1]1/1001/1,0001/10,000Hb=3 m132172201Hb=6 m158205240

Flood Risk AssessmentBenefitsConceptualization of Benefits and Risk 42

Integrated Risk:Barriers protect against a wide range of events up to a predefined probability of occurrence.

Likewise, risk is the integrated risk up to the desired safety level starting from the most frequent event.0.10.0001

Varies for different barrier heights and safety levelMost frequent event: 1/8 yr-1 event (Keim&Muller)42

Flood Risk AssessmentBenefitsDamage reduction43

I found the risk from the integration of these graphs for each barrier height. The risk without the coastal spine was calculated from the integration of the damage-probability curve in slide 31.43

Flood Risk AssessmentDetermination of the optimum safety levelTotal Costs (=Cumulative Risk + Investment)441/1,000-1/10,000

From the graph we can determine the area of the safety levels within which we can move. Obviously, below 1/1,000 yr-1 safety level, the risk without the barriers is less than the total costs of the structure. Thus, it is not cost efficient to choose a safety level below this one. Of course, here we assumed a land barrier that fully blocks the surge. If overflow of the land barrier is considered then costs might decrease, but this requires further investigation.

This graph is from a discounting over 100 years

Safety level refers to the probability of exceedance of the water level in the gulf side


Flood Risk AssessmentDetermination of the optimum safety levelNet Benefits & Benefit-Cost ratio45

Discounted over 50 yrDiscounted over 100 yr

Safety level refers to the probability of exceedance of the water level in the gulf side


Flood Risk AssessmentDetermination of the optimum safety levelNet Benefits & Benefit-Cost ratio46Discounted over 200 yr

The larger the lifetime the larger the NB and the BC ratio.

Positive NB are noticed from 100 yr lifetime and on, for a barrier height of above 2 m. 1/10,000 yr-1 safety level gives the largest NB.

200 yr lifetime also leads to positive NB but with larger total costs.

1)However, for a larger structure lifetime, initial investment costs would probably be larger. On the other hand, maintenance costs are reduced.2)The proposed safety level (1/10,000 yr-1) also coincides with Kasper Stoetens (2013) results about the optimum safety level.3)100 yr lifetime is the first to give positive NB with the lowest costs.46

ConclusionsOptimal Barrier height (100 yr lifetime)472.6

For 100 yr lifetime, the lower the safety level is the lower the total costs are. However, NB determines the optimum safety level and thus for this level, the optimum barrier height is chosen.47

ConclusionsLifetime: 100 yr? Safety level: 1/10,000 yr-1Optimal barrier height: 2. 6m 4 m+MSL inside the basin for a 1/10,000 yr-1 storm surge level in the gulf side 1/3 inflow and 2/3 wind setupCBA led to height close to 3 m after which water levels inside the basin tend to be almost the same (probabilistic analysis)The higher the barrier the lower the water level inside the basin

48Water level inside the basin [m]Return period [yr-1]Water level in the gulf side [m]Without coastal spineHb=1 mHb=2 mHb=3 mHb=6 m1/1003.,0004.,0005.,0005.

Numbers refer to the conclusions in sequence:1)As structures lifetime increases, risk as well as total costs also increases. In terms of total costs (risk+investment), for 100 yr and 200 yr lifetime the optimum safety level is 1/100 yr-1, for 50 yr is 1/5,000 yr-1. However, the value of the net benefits determines the optimum safety level. We can compare the total costs of each alternative to the status quo (doing nothing alternative) in order to decide if it is worthwhile to undertake a specific alternative (total costs of alternative< risk of doing nothing alternative). The chosen lifetime depends also on the B/C that US uses for such investments.5) However, the difference is not significant and that is why in the end a height of 6 m is not beneficial as the risk reduction does not overweigh its investment costs48

ConclusionsMaintenance costs 6% of the investmentsBenefits for 2.6 m height (100 yr lifetime)

Lower premiums49

Numbers refer to the conclusions in sequence:6) Calculated for a barrier of 3 m, safety level 1/10,000 yr-1 and 100 yr lifetime7) Lower premiums due to risk reduction: more clients for being insured, development in the new protected areas, win-win situation (better protection, lower insurance for homeowners)49

ConclusionsLower premiums Texas: flood insurance collected is much lower than the flood claims paid 50

Source: Michel-Kerjan, 2010

Low BC ratio? Then this picture might help reconsider the until now benefit cost ratio and probably NFIP will increase the available budget for Texas in terms of flood protection measures. 50

Recommendations Detailed investigation of direct and indirect damageMore precise estimation of the benefits Split Galveston bay region into zones according to their vulnerability and their existing protection levelRisk assessment based also on the individual risk (life loss)More detailed calculation of the investment and maintenance costs (to be estimated after design)Interest and growth rate estimation (constant?)Combinations of different heights of the storm surge with the land barrier heightsReliability issues probability of failure of the barrier?independent hurricane events?

51(additional to the halfway recommendations)

2) Benefits: i.e. more development in the surrounding areas due to safer standards, multiple functions of the coastal spine (road, recreational area), lower insurance3) In order to make a better estimation of the potential damage. Separate areas with different flood protection level from the main regions (i.e. Texas city, port of Houston, Kemah etc) 4) Rapidly increasing population5) Issues like: bed protection, unforeseen costs, costs due to an increased lifetime of the structure (new materials (concrete, steel etc), innovative design), maintenance6) Also different heights of the dike should be investigated in combination with the barriers height 8) In this research, hurricane events were considered independent concerning their effect on the barrier. For each hurricane, barrier maintains its initial strength. However, after repeated storm events it will gradually loose its strength (not only from storm events but also from natural phenomena like corrosion etc.). In such a case, either you accept the decrease in the strength and thus the risk increases or a maintenance procedure takes place which consequently increases the investment costs.51

Flood Risk Assessment Methodology 52Surge elevation inside the basinTopography elevation Inundation mapsResidential exposureIndustrial exposureResidential property valueIndustrial property valueDepth Damage curvesFlood Damage Assessment